The present invention relates generally to seals for providing fluid sealing between a housing and a rotating shaft. More particularly, the invention relates to mechanical face seals in which a fluid is introduced between portions of the seal faces of the seal.
Conventional mechanical seals are employed in a wide variety of mechanical apparatuses to provide a pressure-tight and a fluid-tight seal between a rotating shaft and a stationary housing. The seal is usually positioned about the rotating shaft, which is mounted in and protrudes from the stationary housing. The seal is typically bolted to the housing at the shaft exit, thus preventing loss of pressurized process fluid from the housing. Conventional mechanical seals include face type mechanical seals, which include a pair of annular sealing rings that are concentrically disposed about the shaft, and axially spaced from each other. The sealing rings each have seal faces that are biased into physical contact with each other. Usually, one seal ring remains stationary, while the other ring contacts the shaft and rotates therewith. The mechanical seal prevents leakage of the pressurized process fluid to the external environment by biasing the seal ring sealing faces into physical contact with each other. As a result of the repeated physical contact between the faces, abrasion of the seal faces occurs and the seals typically exhibit undesirable wear characteristics and leakage.
The prior art attempted to overcome the above difficulties by employing non-contact mechanical seals that utilize a fluid interposed between the seal ring faces to reduce frictional wear. Conventional mechanical non-contact face seals typically employ pumping grooves, such as spiral or Raleigh step grooves, formed in one of the seal faces of the seal rings to develop a hydrodynamic lifting force that separates the seal faces. The resultant gap allows fluid to be disposed between the seal faces to prevent rubbing and abrasion of the seal faces.
Conventional non-contacting face seals, however, exhibit drawbacks in some areas of performance that are less than optimal. For example, in non-contacting seal designs, which rely primarily upon rotation to provide hydrodynamic separation of the seal faces, a substantial amount of seal face abrasion can occur during start-up operation or during periods when the shaft is rotating at low speeds. For this reason, these conventional non-contacting type mechanical face seals are unsuitable for low speed operation or for conditions which require frequent starting and stopping of the shaft.
In order to overcome the problems associated with purely hydrodynamic non-contacting seals, combination hydrostatic and hydrodynamic seals have been designed. Such combination seals typically rely on fluid pressure to provide hydrostatic separation and rotation to provide hydrodynamic separation of the seal faces. The performance of such combination seals has been less than optimal because the thickness of the fluid gap formed between the seal faces varies significantly depending upon rotation speed. Such conventional combination mechanical seals exhibit a substantial difference in fluid film thickness between hydrostatic (i.e., non-rotating) and hydrodynamic (rotating) operation due to the significant pumping force provided by the spiral grooves. The larger gap formed between the seal faces at high rotation speeds allows for greater leakage across the seal faces than would otherwise be desirable.
A significant number of conventional non-contacting seals employ a dual seal arrangement in which three or more seals rings are arranged axially along the shaft. Such dual seals can be arranged in a back-to-back, face-to-face, or tandem configuration and typically employ a high pressure barrier fluid at the outer diameter of the seal rings while maintaining the process fluid at the inner diameter of the seal rings. The barrier fluid is introduced to the seal faces through pumping grooves formed in one of the seal faces.
Dual non-contacting seals have also proven to be less than optimal in an number of areas of performance. Sealing the process fluid at the inner diameter of the seal can result in dirt or other particles within the process fluid becoming clogged between the seal faces, interfering with the operation of the seal. In addition, upon loss of the barrier fluid pressure, some dual seal designs do not maintain a fluid-tight seal, resulting in leakage of the process fluid. The additional seals rings in dual seals also result in an exceedingly bulky seal that is often unsuitable for applications in which the axially space along the shaft is limited. Furthermore, dual seals typically require modification of the housing to accommodate the increased size of the seal, resulting in complex and costly installation and servicing of the seal.
As the above described and other prior art seals have proven less than optimal, an object of the present invention is to provide an improved non-contacting mechanical face seal that is operable under a wider range of operating conditions.
Another object of the present invention is to provide a non-contacting mechanical face seal that maintains a fluid-tight seal that is less dependent on shaft speed.
Still another object of the present invention is to provide a non-contacting mechanical seal that minimizes seal face contact at lower shaft speeds and is suitable for applications requiring frequent starting and stopping of the shaft.
Yet another object of the present invention is to provide a non-contacting mechanical face seal that is compact in design and can be installed without modification of the housing.
A further object of the present invention is to provide a non-contacting mechanical seal that can provide the benefits of hydrostatic and hydrodynamic operation simultaneously.
Another object of the present invention is to provide a non-contacting mechanical seal having a sealing structure that alleviates O-ring hysteresis.
Other general and more specific objects of this invention will in part be obvious and will in part be evident from the drawings and the description which follow.
These and other objects of the present invention are attained by a mechanical seal of the present invention which provides fluid sealing between a housing and a rotatable shaft and is suitable for operation over a wide range of operating conditions, including at low shaft speeds. The seal is preferably a non-contacting seal that provides for hydrostatic operation over a portion of one of the seal faces and hydrostatic and hydrodynamic operation over another portion of the seal faces. Accordingly, the mechanical seal of the present invention allows for partial or complete separation of the seal faces independent of shaft speed by having a portion of the seal faces exposed solely to a hydrostatic fluid force. Thus, contact between the seal faces at start-up or at low shaft speeds can be minimized or eliminated thereby reducing wear on the seal faces. Additionally, the mechanical seal of the present invention provides the advantages of hydrodynamic operation at higher shaft speeds, thereby increasing the overall range of effective operating conditions for the seal.
In a preferred embodiment, the mechanical seal of the present invention includes a first seal ring having a first seal face and a second seal ring having a second seal face. The first seal face further has a first portion and a second portion. The seal faces of the first and second seal rings are opposed to each other when assembled. One of the seal rings is adapted to rotate with the rotating shaft, and the other seal ring is restrained from rotating. The seal faces are configured to produce a primarily hydrostatic fluid force between at least a portion of the first portion of the first seal face and at least a portion of the second seal face. In addition, the seal faces are configured to produce a hydrodynamic fluid force and a hydrostatic fluid force between at least a portion of the second portion of the first seal face and at least a portion of the second seal face.
Preferably, the first seal ring has a first outer radially extending seal face at the first portion of the first seal face and a second inner radially extending seal face at the second portion of the first seal face. The first outer seal face and the second inner seal face are generally co-planar. Preferably, the second seal face is sized to overlap at least a portion of the inner and outer seal faces of the first seal ring, thus the seal rings are capable of generating the hydrostatic and the hydrodynamic forces as a result of dam portions formed by the seal face overlap. The first outer seal face can be disposed along an outer circumferential portion of the first seal face and the second inner seal face can be disposed along an inner circumferential portion of the first seal face to form a dual concentric seal on a single seal ring.
The mechanical seal of the present invention preferably employs a plurality of pumping grooves formed in the second portion of the first seal face to produce the hydrodynamic fluid force between at least a portion of the second portion of the first seal face and at least a portion of the second seal face. Barrier fluid can be introduced to the plurality of pumping grooves formed in the first seal face such that the pumping grooves and the fluid generate the hydrodynamic and the hydrostatic fluid forces between the first and second seal faces to separate selectively at least a portion of the first seal face from at least a portion of the second seal face.
In a preferred embodiment, a plurality of passages can be formed within the second seal ring to introduce barrier fluid to the pumping grooves formed in the first seal face. Each passage can open onto the second seal face at one end and can be in fluid communication with a fluid source at another end. A circumferential groove can also be formed in the second seal face and can be positioned on the second seal face such that the passages open onto the circumferential groove. The circumferential groove and the passages are preferably in registration with at least a portion of the pumping grooves formed in the first seal face, such that the passages and the circumferential groove provide fluid to the pumping grooves to generate the hydrodynamic fluid force.
In a preferred embodiment, the seal of the present invention can also include a fluid control system for controlling the separation of the seal faces by adjusting the pressure of the barrier fluid introduced to the grooves. Preferably, the fluid control system adjusts the thickness of the gap formed between the seal faces by adjusting the barrier fluid pressure over the process fluid pressure during operation of the seal.
The mechanical seal of the present invention can include a sleeve for securing the first seal ring to the rotating shaft. The sleeve has a flanged end and is sized for mounting generally concentrically about the rotating shaft. The seal can also include an annular lock ring mounted concentrically about the sleeve for securing the sleeve, and thus the first seal ring, to the rotating shaft. The lock ring can include a plurality of apertures formed therein for receiving fasteners which frictionally engage the rotating shaft to secure the lock ring and the sleeve to the shaft.
The mechanical seal of the present invention can also include a gland assembly sized for mounting to the housing and about the shaft. The gland assembly can be coupled to the second seal ring to connect the second seal ring to the housing and, thus, restrain the second seal ring from rotating. The gland assembly can include an axial inner gland plate and an axial outer gland plate. A resilient member, such as an O-ring, can be interposed between the inner gland plate and the outer gland plate to form a seal therebetween.
A resilient member, such as a an O-ring, can be interposed between the second seal ring and the gland assembly to provide a seal between the second seal ring and the gland assembly. A compression member can also be provided for axially and radially biasing the resilient member into contact with the second seal ring and the gland assembly. The compression member is preferably an annular compression plate having an annular inner flanged portion for engaging the resilient member. The inner flanged portion can include an axially and radially extending angled surface for axially and radially biasing the resilient member into contact with the second seal ring and the gland assembly.
The mechanical seal of the present invention can optionally include a system for introducing a closing fluid to a rear surface of the second seal ring to provide a closing force on the second seal ring. The closing force preferably acts upon a portion the second seal face overlapping the second portion of the first seal face. The closing fluid system can include a fluid conduit formed in the outer gland plate that opens proximate the rear surface of the second seal ring at one end and is in fluid communication with a fluid source at another end. Preferably, a common fluid supply provides both the closing fluid to the closing fluid system and barrier fluid for introduction to the seal faces.